Cascade Refrigeration Loops to Prevent Pump Cavitation
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Solution Overview
Problem
Conventional refrigerant systems using CO2 as a heat transfer fluid face equipment and operational complexity due to the need for maintaining it in a supercritical state, which can lead to cavitation and instability in pumps, especially when subcooling is eliminated to reduce power consumption.
Innovation Solution
A cascade air conditioning system with two-phase refrigerant loops, where a second heat transfer fluid with specific toxicity and flammability ratings is used, and a liquid pump inlet is subcooled between 0° C. and 10° C. to prevent cavitation, reducing power consumption by 1-2% annually.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If CO2 is maintained in a supercritical fluid state to eliminate ozone depletion and reduce global warming potential, then environmental performance is improved, but equipment complexity and operational complexity increase
Solution Approach 1:
The system is divided into two separate loops: a first loop using CO2 as heat transfer fluid for heat rejection, and a second loop using a different heat transfer fluid (with ASHRAE Class A toxicity and Class 1 or 2L flammability ratings) for heat absorption. This segmentation allows each loop to be optimized independently, with the CO2 loop operating at high pressure for efficient heat rejection while the second loop uses a fluid better suited for evaporative cooling, thereby reducing overall system complexity while maintaining environmental performance.
Solution Approach 2:
A heat exchanger serves as an intermediary between the two loops, transferring heat from the CO2 loop to the second heat transfer fluid loop. This intermediary allows the system to leverage the advantages of CO2 (low GWP, zero ODP) for heat rejection while using a different fluid for heat absorption, avoiding the need to maintain CO2 in a complex supercritical state throughout the entire system.
2Use of energy by moving object
If subcooling is eliminated at the pump inlet to reduce power consumption, then energy efficiency is improved, but pump stability deteriorates due to cavitation
Solution Approach 1:
The system changes the physical parameters of the heat transfer fluid by using a different fluid in the second loop with specific properties (ASHRAE Class A toxicity and Class 1 or 2L flammability ratings) that allow for reduced subcooling requirements. This parameter change enables the pump to operate stably with minimal or no subcooling, reducing power consumption while maintaining reliability.
Solution Approach 2:
The system utilizes phase transitions of the heat transfer fluids at different points in the cycle. The CO2 undergoes phase change in the heat rejection heat exchanger, while the second heat transfer fluid undergoes phase change in the heat absorption heat exchanger. This controlled use of phase transitions allows the system to manage fluid states optimally, preventing cavitation at the pump inlet while minimizing the need for excessive subcooling.
3Reliability
If CO2 is subcooled upstream of the pump inlet to force complete phase change to liquid, then pump cavitation is prevented, but power consumption increases
Solution Approach 1:
The system segments the refrigeration function into two separate loops, with the CO2 confined to the heat rejection loop where subcooling is not required. The second loop handles the evaporative cooling function with a fluid that does not require excessive subcooling, thereby eliminating the need for energy-intensive subcooling of CO2 while maintaining pump stability.
Solution Approach 2:
The heat exchanger acts as an intermediary that transfers heat from the CO2 loop to the second heat transfer fluid loop. This allows the system to avoid subcooling CO2 by transferring its thermal energy to the second fluid, which then undergoes phase change in the evaporator, reducing power consumption while maintaining reliable pump operation.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system stabilizes pump operation, reduces power consumption, and meets global warming and ozone depletion potential targets by using a secondary heat transfer fluid with ASHRAE Class A toxicity and Class 1 or 2L flammability ratings, enhancing thermal conductivity with nanoparticles.
Implementation Method 1
a heat rejection side of the heat exchanger evaporator/condenser... transfers heat to the first heat transfer fluid circulation loop through the heat exchanger evaporator/condenser
Implementation Method 2
a liquid pump inlet subcooling is between 0° C. and 10° C.... stabilizes pump operation
Implementation Method 3
heat transfer fluid is compressed in a compressor from a lower to a higher pressure
Implementation Method 4
high-pressure heat transfer fluid flows to an expansion device where it is expanded to a lower pressure and temperature
Implementation Method 5
heat transfer fluid cools a secondary fluid to be delivered to the conditioned environment
Data Source
AI summary
A heat transfer system includes a first two-phase heat transfer fluid vapor/compression circulation loop including a compressor, a heat exchanger condenser, an expansion device, and a heat absorption side of a heat exchanger evaporator/condenser. A first conduit in a closed fluid circulation loop circulates a first heat transfer fluid therethrough. A second two-phase heat transfer fluid circulation loop transfers heat to the first heat transfer fluid circulation loop through the heat exchanger evaporator/condenser, including a heat rejection side of the heat exchanger evaporator/condenser, a liquid pump, a liquid refrigerant reservoir located upstream of the liquid pump and downstream of the heat exchanger evaporator/condenser, and a heat exchanger evaporator. A second conduit in a closed fluid circulation loop circulates a second heat transfer fluid therethrough having an ASHRAE Class A toxicity and a Class 1 or 2L flammability rating.